database/sql: add support for returning cursors to client

This CL add support for converting a returned cursor (presented
to this package as a driver.Rows) and scanning it into a *Rows.

Fixes #28515

Change-Id: Id8191c568dc135af9e5e8555efcd01987708edcb
Reviewed-on: https://go-review.googlesource.com/c/145738
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -1338,6 +1338,52 @@ func TestConnQuery(t *testing.T) {
 	}
 }
 
+func TestCursorFake(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
+	defer cancel()
+
+	exec(t, db, "CREATE|peoplecursor|list=table")
+	exec(t, db, "INSERT|peoplecursor|list=people!name!age")
+
+	rows, err := db.QueryContext(ctx, `SELECT|peoplecursor|list|`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rows.Close()
+
+	if !rows.Next() {
+		t.Fatal("no rows")
+	}
+	var cursor = &Rows{}
+	err = rows.Scan(cursor)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cursor.Close()
+
+	const expectedRows = 3
+	var currentRow int64
+
+	var n int64
+	var s string
+	for cursor.Next() {
+		currentRow++
+		err = cursor.Scan(&s, &n)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if n != currentRow {
+			t.Errorf("expected number(Age)=%d, got %d", currentRow, n)
+		}
+	}
+	if currentRow != expectedRows {
+		t.Errorf("expected %d rows, got %d rows", expectedRows, currentRow)
+	}
+}
+
 func TestInvalidNilValues(t *testing.T) {
 	var date1 time.Time
 	var date2 int